1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a printer and, more particularly, to an improvement in the image forming apparatus of the mode in which a toner image on an image carrier is electrostatically transferred to a sheet on a sheet conveyor and in which the sheet is separated from the sheet conveyor.
2. Description of the Related Art
In a color image forming apparatus of the so-called "tandem type", as known in the prior art, multiple image forming units (adopting the electrophotographic method, for example) are arranged a sheet conveying passage extending in a horizontal direction, for example, so that toner images are sequentially transferred from the individual image forming units to a sheet (a transfer material such as paper or OHP sheet) moving along the sheet conveying passage to form a color image on the sheet.
As the sheet conveying method of the color image forming apparatus of this kind, there has already been proposed (as referred to Unexamined Published Japanese Patent Application No. 5-53412, for example) a belt conveyor method in which a conveyor belt circulating along the sheet conveying passage is provided, for example, for adsorbing and holding a sheet electrostatically on the conveyor belt.
Here in the tandem type color image forming apparatus of this kind, the conveyor belt is made to run on a plurality of (e.g., two) tension rolls, and the running portion of the conveyor belt on the tension rolls is formed into a branch passage curved with respect to the linear sheet conveying passage so that the sheet on the conveyor belt is naturally separated by using a portion of the running portion of the conveyor belt on the tension roll as a sheet separating portion.
At this time, when the sheet is to be separated from the conveyor belt, the so-called "separation discharge" occurs at the separation portion between the sheet and the conveyor belt. As a result, the charge state on the back face of the sheet abruptly changes to raise a technical problem that the toner on the sheet disperses.
Especially in the tandem type image forming apparatus adopting the belt conveyor method, the sheet is electrostatically adsorbed by the conveyor belt, and the sheet and the conveyor belt are sequentially charged at a transfer step by the individual image forming units. As a result, the sheet and the conveyor belt are liable to be highly charged to cause the aforementioned toner dispersion phenomenon seriously.
In order to solve this technical problem, there can be enumerated a method in which a destaticizing member is arranged on the back side of the conveyor belt and in front of and in contact with the sheet separating portion to reduce the charge of the conveyor belt. Even if this method is adopted, a toner image in a charged state is carried on the sheet, and the charged state of the conveyor belt can not be completely eliminated. The cause for the separation discharge still remains when the sheet is to be separated from the conveyor belt, so that the separation discharge at the sheet separating time is not completely eliminated.
In order to solve this technical problem, there has already been proposed a technique (as referred to Unexamined Published Japanese Patent Application No. 7-271200, for example) in which an electrically grounded conductive guide plate is arranged in the vicinity of the sheet separating portion of the conveyor belt and within a range of 5 mm with respect to the conveyor belt so that the charge on the sheet may be slowly discharged through the conductive guide plate to prevent the separation discharge at the sheet separation time.
In this time, too, the sheet separating portion on the conveyor belt and the conductive guide plate are arranged at a spacing. At the start of the sheet separation, therefore, the separation discharge may occur by the time the leading edge of the sheet comes into contact with the conductive guide plate so that the technique is still insufficient for eliminating the toner dispersion phenomenon by the separation discharge.
As shown in FIG. 30, on the other hand, there has already been known a technique (as referred to Unexamined Published Japanese Patent Application No. 6-230681 or Unexamined Published Japanese Patent Application No. 9-258565, for example). In an image forming apparatus of the mode in which a transfer roll 302 (for applying a transfer bias 303) is arranged in forced contact with a drum-shaped photosensitive member 301, for example, in order to prevent the phenomenon in which a sheet 306 having passed through the nipping region between the photosensitive member 301 and the transfer roll 302 sticks to the photosensitive member 301, a sheet discharging electrode 304 having saw teeth, for example, for discharging the back face of the sheet 306 is arranged in the vicinity of the downstream of the nipping region, and a predetermined bias 305 is applied to the sheet discharging electrode 304 to cause the discharge between the sheet 306 and the sheet discharging electrode 304 thereby to destaticize the sheet 306, so that the electrostatic adsorption of the sheet 306 to adsorb the photosensitive member 301 is eliminated to separate the sheet 306 from the photosensitive member 301.
According to these techniques, however, the back face of the sheet 306 is destaticized to separate the sheet 306, as having stuck to the side of the photosensitive member 301. In the aforementioned belt conveyor type image forming apparatus, however, the sheet passes through the photosensitive member portions of the individual image forming units while being electrostatically adsorbed by the conveyor belt. From the standpoint of preventing the stick to the photosensitive member, therefore, it is intrinsically of little necessity to arrange the aforementioned sheet discharging electrode 304.
If this sheet discharging electrode 304 is to be arranged, in the mode in which the sheet discharging electrode is arranged on the surface side of the sheet, the toner image on the sheet surface is directly disturbed by the discharge from the sheet discharging electrode. In the mode in which the sheet discharging electrode is arranged on the back side of the sheet, the presence of the conveyor belt makes it difficult to destaticize the sheet so that the separation discharge to be caused when the sheet is separated from the conveyor belt cannot be eliminated to make it difficult to prevent the toner dispersion from occurring at the separation.
In order to solve this technical problem, it is conceivable to provide the aforementioned belt conveyor type image forming apparatus, for example, with the sheet discharging electrode downstream of the sheet separating portion of the conveyor belt and on the back side of the sheet so that the discharge may be caused between the sheet and the sheet discharging electrode to destaticize the sheet.
In this mode, however, the separation discharge at the sheet separating time may be reduced, but the toner dispersion may be caused by the discharge between the sheet and the sheet discharging electrode.
Here, this technical problem should not be limited to the aforementioned belt conveyor type image forming apparatus but will also occur in the image forming apparatus of the mode in which the transfer roll is arranged in forced contact with a drum-shaped photosensitive member, for example. In the image forming apparatus of the mode in which the transfer roll is arranged in forced contact with the drum-shaped photosensitive member, more specifically, the separation discharge occurs when the sheet having passed through the nipping region between the photosensitive member and the transfer roll is separated from the transfer roll, and the toner dispersion may occur on the sheet.